Synthetic graphites are widely used as standard negative electrode materials in lithium ion batteries. Other carbonaceous materials are also widely used in such batteries due to their efficiency and reasonable cost. Lithium ion batteries are primarily used as power sources in portable electronic devices. Compared to other classes of rechargeable batteries such as nickel-cadmium and nickel-metal hydride storage cells, lithium ion cells have become increasingly popular due to relatively high storage capacity and rechargeability.
Due to increased storage capacity per unit mass or unit volume over similarly rated nickel-cadmium and nickel-metal hydride storage cells, the smaller space requirements of lithium ion cells allow production of cells that meet specific storage and delivery requirements. Consequently, lithium ion cells are popularly used in a growing number of devices, such as digital cameras, digital video recorders, computers, etc., where compact size is particularly desirable from a utility standpoint.
Nonetheless, rechargeable lithium ion storage cells are not without deficiencies. These deficiencies may be minimized with the use of improved materials of construction. Commercial lithium ion batteries which use synthetic graphite electrodes are expensive to produce and have low relatively lithium capacities. Additionally, graphite products currently used in lithium ion electrodes are near their theoretical limits for energy storage (372 mAhr/g). Accordingly, there is a need in the art for improved electrode materials that reduce the cost of rechargeable lithium batteries and provide improved operating characteristics, such as higher energy density, greater reversible capacity and greater initial charge efficiency. There also exists a need for improved methods for the manufacture of such electrode materials.
Silicon has been investigated as an anode material for lithium ion batteries because silicon can alloy with a relatively large amount of lithium, providing greater storage capacity. In fact, silicon has a theoretical lithium capacity of more than ten times that of graphite. However, pure silicon is a poor electrode material because its unit cell volume can increase to more than 300% when lithiated. This volume expansion during cycling destroys the mechanical integrity of the electrode and leads to a rapid capacity loss during battery cycling. Although silicon can hold more lithium than carbon, when lithium is introduced to silicon, the silicon disintegrates and results in less electrical contact which ultimately results in decreased ability to recharge the storage cell.
Continuous research efforts in solving silicon volume expansion problems have yielded limited results. Silicon/carbon composite particles or powders have good cycle life compared to mechanical mixtures of carbon and silicon powders made by milling or other mechanical methods. Thin film silicon-coated carbon particles or carbon-coated silicon powders are potential replacements for graphite powders as the anode material for next generation lithium ion batteries. However, chemical vapor deposition methods typically used to apply silicon coatings or carbon coatings have intrinsic shortcomings that include slow deposition rates and/or expensive precursors for deposition. Vapor deposited silicon films may be extremely expensive relative to the cost of bulk silicon powders. Therefore, another method of manufacturing coated silicon particles is needed.